1. Field of the Invention
This invention relates to a photoelectric converter and more particularly to a photoelectric converter capable of amplifying a photoelectrically converted power, which comprises a light-receiving section and a light-shielding section, or a photoelectric converter of a semiconductor transistor, which comprises two semiconductor regions of same electroconductive type and a semiconductor region of opposite electroconductive type to that of the two semiconductor regions, where the semiconductor region of opposite electroconductive type is irradiated with light and an amplified power is output from at least one of the two semiconductor regions of same electroconductive type.
2. Related Background Art
A photoelectric converter which accumulates, a charge generated by light irradiation and outputs an amplified power corresponding to the accumulated charge is one type of photoelectric converters for use in a picture reading device in a facsimile machine, a copying machine, etc. One particular type of such photoelectric converters is a photoelectric converter which accumulates carriers (holes) in a base region of a phototransistor by light irradiation of the base region and outputs an amplified electric current from an emitter region, where the base region of the phototransistor is formed as a light-receiving section and the emitter region as a light-shielding section. A photoelectric converter using a phototransistor can amplify carriers in the light-receiving region, increase sensitivity, reduce random noise and increase S/N ratio, as compared with a photoelectric converter using a photodiode and having no function to amplify a photoelectric current.
FIG. 1 is a schematic cross-sectional view of a conventional photoelectric converter, where numeral 1013 is an n-type layer functioning as a collector of a phototransistor, 1003 is an n-epitaxial layer functioning, 1005 is a p layer as a base region of the phototransistor, 1006 is an n layer functioning as an emitter region of the phototransistor, 1007-2 is an emitter electrode made of Al, etc., 1008 is a LOCOS oxide film, L is a light-receiving section and D is a light-shielding section.
When such photoelectric converters are used in a color line sensor 1101, as shown in FIG. 2, three line sensors are provided for red (R), green (G) and blue (B), where each line sensor comprises a light-shielding section 1011 and a light-receiving section 1012. The length of the light-shielding section 1011 and that of the light-receiving section 1012 in the line sensor arrangement direction, i.e., direction A in FIG. 2, are n bits and one bit, respectively. One bit corresponds to the light-receiving section of one sensor cell and is in a rectangular shape, whose one side is, for example, 10 .mu.m long.
The respective line sensors corresponding to R, G and B are positionally different from one another, and thus when a signal R is obtained at a given line of a draft, a signal G is a signal at a position of the (n+2)th line from the given line and a signal B is a signal at a position of the (2n+3)th line from the given line. In order to obtain signals R, G and R at the same position of the draft, at least the previously output signals G and B must be stored in an external memory 102 through the respective sample hold circuits (S/H) 1014 and A/D conversion circuits 1015 among the signals R, G and B.
Such photoelectric converters are disclosed, for example, in Laid-Open European Patent Application No. 0132076.
FIG. 3A is a schematic plan view showing the structure of a photoelectric converter in detail and FIG. 38 is a schematic cross-sectional view showing the line A--A' of FIG. 3A, where photoelectric converter cells are arranged on an n silicon substrate 3201, and each cell is electrically insulated from the adjacent cells through device-separating regions 3202 comprising SiO.sub.2, Si.sub.3 N.sub.4, polysilicon or the like. Each cell has the following structure. A p base region 3204 and a p region 3205 are formed on an n.sup.- region 3203 with a low impurity concentration, formed by an epitaxial technique, etc., through doping with p-type impurity such as boron, etc., and an n.sup.+ emitter region 3206 is formed on the p base region 3204. The p base region 3204 and the p region 3205 also act as a source and a drain of a p channel MOS transistor as will be described later.
An oxide film 3207 is formed on the n.sup.- region 3203 having the thus formed subregions, and a gate electrode 3209 and a capacitor electrode 3208 of the MOS transistor are formed on the oxide film 3207. The capacitor electrode 3209 is counterposed to the p base region 3204 through the oxide film 3207 to form a capacitor for controlling the base potential.
In addition, an emitter electrode 3210 connected to the n.sup.+ emitter region 3206 and an electrode 3211 connected to the p region 3205 are further formed, and a collector electrode 3212 is formed on the back side of the substrate 3201 through an ohmic contact layer.
The operations of the photoelectric converter cell will be described below. Light is input into the photoelectric converter cell from the side of the p base region 3204 and carriers (in this case holes) are accumulated in the p base region 3204 in an amount corresponding to the light quantity (accumulation operation).
The base potential changes with the accumulated carriers and an electrical signal corresponding to the input light quantity is read out by reading the potential change through the emitter electrode 3210 (reading operation).
A refresh operation to remove the holes accumulated in the p base region 3204 will be described below.
FIGS. 4A and 4B show potential wave forms for explaining the respective refresh operation. As shown in FIG. 4A, an MOS transistor is brought into an ON state only when a negative potential higher than the threshold value is applied to the gate electrode 3208. As shown in FIG. 4B, the emitter electrode 3210 is earthed and the electrode 3211 is brought to an earth potential to carry out the refresh operation. Then, a negative potential is applied to the gate electrode 3208 at first to put the p channel MOS transistor in an ON state, whereby the potential of p base region 3204 can be kept at a constant value, irrespective of the accumulated potential level. Then, a positive potential pulse for refresh operation is applied to the capacitor electrode 3209, whereby the p base region 3204 is biased in the forward direction to the n.sup.+ emitter region 3206 and the accumulated holes are removed through the earthed emitter electrode 3210. At the time of rising of the refresh pulse, the p base region 3204 is returned to the initial state of negative potential (refresh operation). After the potential of the p base region 3204 is made constant by the MOS transistor, the residual charge is erased by the application of the refresh pulse in this manner, and thus a fresh accumulation operation can be carried out again, independently of the previously accumulated potential. Furthermore, the residual charge can be rapidly erased and thus a high speed operation can be carried out. Thereafter, the accumulation operation, reading operation and refresh operation are likewise repeated.
The potential Vp generated in the base by the holes accumulated in the base by photoexcitation can be given by the following formula: EQU Vp=Q/C
where Q is a charge amount of holes accumulated in the base and C is a capacitance connected to the base. As is obvious from the formula, a high level of integration can reduce the cell size and also reduce both Q and C, and thus the potential Vp generated by photoexcitation can be kept substantially constant. Thus, the system proposed above is also advantageous for higher resolution. However, a higher blue sensitivity and a higher response speed of the semiconductor transistor have been sometimes required for the foregoing photoelectric converter and thus a further improvement of the characteristics has been desired.
When a picture treatment is carried out with the color line sensor 1101 as explained, referring to FIGS. 1 and 2, a higher capacity of the external memory than a given one is required for storing the previously output signals G and B in the external memory so as to obtain signals R, G and B at the same position of a draft, and a reduction of the necessary memory capacity has been desired on account of cost reduction, etc. Furthermore, there has been a problem of difficult optimization in the base region of a phototransistor because the optimum conditions of the size, impurity concentrations, etc. in the photoelectric conversion region are different from those for a bipolar transistor, and a further improvement thereof has been also desired.